In 3GPP (3rd Generation Partnership Project), studies are being carried out on LTE-advanced. In LTE-advanced, the introduction of a band extension technique called “carrier aggregation” is under study.
In carrier aggregation, a plurality of component carriers (CCs), one CC being composed of 20 MHz, are bundled to aggregate a plurality of carriers and an approach for realizing high-speed transmission is adopted for downlink (DL) and uplink (UL) channels. In LTE-advanced, studies are being carried out with the introduction of five CCs, that is, band extension of up to 100 MHz in view.
In line with this trend, a transmission power control method targeting UL carrier aggregation is also being studied at the same time. In the design of UL transmission power control, a plurality of matters as shown below need to be considered.
First, a limit value is placed on transmission power of a terminal and values satisfying legal standards (e.g., maximum transmission power value) are set in each country.
Second, transmission power control of a UL channel (PUSCH: Physical Uplink Shared CHannel) of 3GPP LTE Rel.8 is a control method using both closed-loop control and open-loop control, and therefore a pathloss value used for setting UL transmission power is calculated through estimation from a reference signal received by a terminal on a DL. Therefore, a base station generally does not know an accurate pathloss value used for setting UL transmission power.
Third, a value dependent on the implementation of the terminal which is unknown to the base station (e.g., maximum power reduction (MPR) which is a value dependent on the implementation of RF of the terminal) also has an influence on the actual transmission power value of the terminal.
In consideration of such matters, in order for the base station to perform flexible scheduling in UL transmission, time-frequency resource allocation and link adaptation (adaptive modulation, channel coding, closed-loop transmission power control or the like), the base station needs information to know actual transmission power of the terminal.
Thus, conventionally (3GPP LTE Rel.8), the terminal reports, to the base station, information on a transmission power headroom called “PHR” at the terminal using a UL channel (to be more specific, PUSCH or the like). A PHR of 3GPP LTE Rel.8 (following equation 2) is defined by a difference between maximum transmission power of the terminal and a transmission power value of a PUSCH (following equation 1).[1]PPUSCH(i)=min{Pcmax,10 log10(MPUSCH(i))+Po_PUSCH(j)+α(j)PL+ΔTF(j)+f(i)}   (Equation 1)[2]PH(i)=Pcmax−{10 log10(MPUSCH(i))+Po_PUSCH(j)+α(j)PL+ΔTF(j)+f(i)}   (Equation 2)
In equations 1 and 2, Pcmax denotes maximum transmission power for each CC, MPUSCH(i) denotes an allocation bandwidth of a PUSCH, Po_PUSCH(j) and α(j) denote parameters reported from the base station using a higher layer, PL denotes a pathloss estimate value estimated by the terminal, ΔTF(j) denotes an offset value related to a Modulation and channel Coding Set (MCS), and f(i) denotes an accumulated value of a Transmit Power Control (TPC) command.
The terminal reports the PHR to the base station and the base station appropriately performs link adaptation and time-frequency scheduling using the PHR reported from each terminal.
In 3GPP LTE Rel.8, since such PHR reporting targets only one CC of 20 MHz, studies are being carried out on a method of reporting a PHR targeting carrier aggregation made up of a plurality of CCs of the UL. The following are terms of agreement relating to PHR reporting of UL carrier aggregation to date in 3GPP.
As a method of reporting a PHR of UL carrier aggregation, it is agreed that a PHR is reported for each CC and the following two types are defined as PHR for each CC.
Type 1: Pcmax−PUSCH transmission power (P_cmax minus PUSCH power)
Type 2: Pcmax−PUCCH transmission power−PUSCH transmission power (P_cmax minus PUCCH power minus PUSCH power)
When there is no Physical Uplink Control CHannel (PUCCH) transmission, Format 1A is used as a reference format to calculate a PHR. On the other hand, when there is PUCCH transmission, the actual format reported from the base station to the terminal is used to calculate transmission power and a PHR.
Here, Pcmax denotes maximum transmission power for each CC of the terminal. Type 1 mainly targets CCs which will not simultaneously transmit PUCCH and PUSCH, and is defined with a difference value between maximum transmission power for each CC and PUSCH transmission power as in the case of the definition of 3GPP LTE Rel.8.
On the other hand, type 2 mainly targets CCs which may simultaneously transmit PUCCH and PUSCH, and is defined with a value resulting from subtracting a total value of transmission power of PUCCH and PUSCH from maximum transmission power for each CC.
However, two cases are anticipated; a case where there is simultaneous transmission of PUCCH and PUSCH in a CC and a case where there is no simultaneous transmission. For this reason, when there is no PUCCH transmission, Format 1A used for transmission of ACK (Acknowledgment) or NACK (Non-acknowledgment) or the like is used as a reference format to calculate PUCCH transmission power. When there is PUCCH transmission, the actual transmission format reported from the base station is used to calculate PUCCH transmission power. In equation 1 and equation 2, examples of information related to a UL grant (transmission format) reported through a DL control channel of a physical layer (PDCCH: Physical Downlink Control CHannel) include PUSCH allocation bandwidth information (MPUSCH(i)), MCS-related offset value (ΔTF(j)) and accumulated value (f(i)) of TPC command.
Furthermore, as the method for reporting PHRs of a plurality of CCs, it is agreed that when PHR reporting is triggered, PHRs of all CCs configured (Per CC PHR) are fed back to the base station through an UL channel (an approach for selecting a PHR to be fed back is also considered). However, when there is no UL grant (transmission format; UL transmission bandwidth, MCS information or the like) to be reported from the base station to the terminal for all PUSCHs of a plurality of CCs (or a plurality of PUSCHs of CCs for reporting a PHR), it is not possible to calculate transmission power and PHR of a PUSCH with no UL grant (see FIG. 1).
In FIG. 1, at PHR report timing, a per CC PUSCH enclosed by a dotted line denotes a PUSCH with no UL grant (no transmission), a shaded area denotes a PUSCH with a UL grant (with transmission) and a diagonally shaded area denotes a PUCCH. Therefore, the per CC PUSCH enclosed by a dotted line corresponds to a PUSCH using a reference format to calculate the corresponding per CC PHR and the PUSCH of the shaded area corresponds to a PUSCH using a UL grant to calculate the corresponding per CC PHR. Based on 3GPP LTE Rel.8, the PUSCH transmission power and PHR are calculated according to equations 1 and 2. Information such as bandwidth information MPUSCH(i) and ΔTF(j) of equation 1 and equation 2 is calculated based on a (reference) format of a UL grant.
Thus, in reporting of a plurality of per CC PHRs, the definition of a reference format for a PUSCH with no UL grant (no transmission) is currently under study, which is described in NPL 1. As shown in FIG. 2, NPL 1 describes an approach of calculating transmission power and PHR of a current Transmission Time Interval (TTI) using a UL grant in a past TTI reported from a base station to a terminal as a reference format of transmission power of a PUSCH of the current TTI.